This invention relates to improvements in an exhaust gas purifying catalyst for purifying exhaust gas discharged from an internal combustion engine of an automotive vehicle or the like so as to remove HC (hydrocarbons), CO (carbon monoxide) and NOx (nitrogen oxides) from exhaust gas and to a method of producing the exhaust gas purifying catalyst, and more particularly to the exhaust gas purifying catalyst arranged to prevent a catalytic coat layer from being cracked thereby protecting the catalytic coat layer from being peeled off from a substrate such as a ceramic monolithic substrate or a metallic honeycomb-type substrate and to the producing method of such exhaust gas purifying catalyst.
A variety of methods of producing exhaust gas purifying catalysts have been proposed and put into practical use, in which a catalytic coat layer is formed on a monolithic substrate such as a ceramic monolithic substrate or a metallic honeycomb-type substrate. One of them is disclosed in Japanese Patent Provisional Publication No. 53-135898, in which a compound including alumina and ceria carrying noble metals is mixed with an acetic acid aqueous solution and pulverized to form a catalytic slurry. The catalytic slurry was coated on the ceramic monolithic substrate, followed by drying and firing. Another one is disclosed in Japanese Patent Provisional Publication No. 58-122044, in which activated alumina and lanthanum carbonate are added to an aqueous solution of aluminium nitrate, followed by stirring, mixing and pulverization, so as to form a slurry. This slurry is coated on a monolithic substrate, and then an excessive liquid is blown up with compressed air. Thereafter, the coated substrate is dried and fired so as to obtain a coated monolithic substrate. The coated monolithic substrate is dipped in a solution of noble metal(s), followed by drying and firing. A further one is disposed in Japanese Patent Provisional Publication No. 5-285386, in which powder materials such as alumina, cerium and zirconium oxide carrying noble metals are pulverized by a ball mill or the like thereby forming an aqueous slurry. This slurry was coated on a substrate such as a honeycomb-type monolithic substrate.
Now, automotive vehicles have been recently being improved in durability and therefore have become long in usage duration and increased in chance of cruising on freeways with development of freeways. This has required further improvements in durability in exhaust gas purifying catalysts for automotive vehicles.